Advances in the study of molecules have been led, in part, by improvement in technologies used to characterise the molecules or their biological reactions. In particular, the study of the nucleic acids DNA and RNA has benefited from developing technologies used for sequence analysis and the study of hybridisation events.
An example of the technologies that have improved the study of nucleic acids, is the development of fabricated arrays of immobilised nucleic acids. These arrays consist typically of a high-density matrix of polynucleotides immobilised onto a solid support material. See, e.g., Fodor et al., Trends Biotech. 12:19-26, 1994, which describes ways of assembling the nucleic acids using a chemically sensitized glass surface protected by a mask, but exposed at defined areas to allow attachment of suitably modified nucleotide phosphoramidites. Fabricated arrays can also be manufactured by the technique of “spotting” known polynucleotides onto a solid support at predetermined positions (e.g., Stimpson et al., Proc. Natl. Acad. Sci. USA 92:6379-6383, 1995).
A further development in array technology is the attachment of the polynucleotides to the solid support material to form single molecule arrays. Arrays of this type are disclosed in International Patent App. WO 00/06770. The advantage of these arrays is that reactions can be monitored at the single molecule level and information on large numbers of single molecules can be collated from a single reaction.
For DNA arrays to be useful, the sequences of the molecules must be determined. U.S. Pat. No. 5,302,509 discloses a method to sequence polynucleotides immobilised on a solid support. The method relies on the incorporation of 3′-blocked bases A, G, C and T, each of which has a distinct fluorescent label, into the immobilised polynucleotide, in the presence of DNA polymerase. The polymerase incorporates a base complementary to the target polynucleotide, but is prevented from further addition by the 3′-blocking group. The label of the incorporated base can then be determined and the blocking group removed by chemical cleavage to allow further polymerisation to occur.
Welch et al. (Chem. Eur. J. 5(3):951-960, 1999) describes the synthesis of nucleotide triphosphates modified with a 3′-O-blocking group that is photolabile and fluorescent. The modified nucleotides are intended for use in DNA sequencing experiments. However, these nucleotides proved to be difficult to incorporate onto an existing polynucleotide, due to an inability to fit into the polymerase enzyme active site.
Zhu et al. (Cytometry 28:206-211, 1997) also discloses the use of fluorescent labels attached to a nucleotide via the base group. The labelled nucleotides are intended for use in fluorescence in situ hybridisation (FISH) experiments, where a series of incorporated labelled nucleotides is required to produce a fluorescent “bar code”.
WO99/57321 describes the use of nucleotides comprising fluorophores linked to the nucleotide by chemically or photochemically cleavable linker moieties.
WO00/53812 and EP-A2-1 291 354 disclose nucleotide compounds of general structure Fluorophore-S—S-Linker-Nucleotide and their use in nucleic acid assay methods. WO00/53812 also makes reference to periodate cleavage of a cis-glycol linkage between nucleotide and fluorophore.
WO 01/92284 discloses the use of enzyme-cleavable groups linking blocking and reporting moieties to nucleotides. It is preferred that these enzyme-cleavable groups are the same, i.e. that both the blocking and reporter moieties are attached to the nucleotide by a chain comprising a group cleavable by a common enzyme. Cleavable groups described in WO 01/92284 are esters and amides, cleavable by esterases and amidases respectively.
WO02/29003 describes nucleotides analogues that contain a label linked through cleavable linkers to the nucleotide base, or an analogue of the base. Photocleavable linkers comprising 2-nitrobenzyl moieties are described.